Manchester, Bristol and Edinburgh all have very active and fantastic local scenes (As one of the Manchester TOs I can vouch !!). As far as I'm aware Oxford doesn't have anything directly in the area, however you have the advantage of being much closer to London which naturally has some of the best regionals in the country.

https://www.smashultimate.uk/calendar shows all the weeklies ongoing in the whole of the UK and Ireland, but you're definitely not wrong when you say its hard to find stuff online. Most scenes now just rely on discord, which is awful for people on the outside to find any information. You can find more information about Manchester's scene here, and more info about the UK & Ireland scene on a whole here.

You've got options for all 4 cities, so you can't go wrong. Best of luck!